Rejoice! Joshua Dietze of draisberghof.de, that famed creator of usb_modeswitch, has written a fine widget for running USB 3G / EVDO modems on the tablets of ye! It takes all the effort out of the challenge!
The instructions are all on that page. All you need is a tablet with a fully powered USB port, and root access. Of course, the kernel also needs Option driver and PPP support. Download the widget installer and run it for installation. Go to your home screen and insert the widget anywhere (4×1 size) by long-pressing and selecting from the pop-up menu.
The widget works best with Android 3.1 and above, but works alright with Froyo too. Once you insert your modem, usb_modeswitch should run and switch it to modem mode. Press the icon on the widget (the one that says PPP and shows a modem) to get the list of detected modems. Select your modem.
Press the configure button, enter the vital parameters such as the ID and Password associated with your device, the APN if you’re on 3G (EVDO does not usually require an APN), and the correct number to be dialed (#77& for EVDO).
Click Connect, and you have internet access!
Of course, there is still a constraint to connecting to the internet through a USB modem on Android: the Android framework does not recognise the method, although the underlying subsystem works fine with it. This means that some applications (most of Google Apps and Market downloads) that check a connection state variable of Android will not realise that a connection actually exists, and will return an error of no network access. However, many email clients (not the inbuilt ones) and other apps just work fine.
So just go ahead, install the widget and test it! Also thank the community-minded Free Software developers that make all this goodness available to us.